(1) Field of the Invention
The present invention relates to a degradation estimating apparatus for an unburned fuel component adsorption (storage) catalyst.
(2) Description of the Related Art
So far, there has been employed a catalyst which is for reducing an exhaust gas exhausted from an engine to the atmosphere. There is a problem which arises with the employment of a catalyst, however, in that the catalyst gradually degrades to show a lower purification performance. For this reason, there is a need to timely estimate the purification performance of the catalyst, i.e., the degree of degradation of the catalyst.
As one of the technologies for the estimation on the degree of degradation of the catalyst, there is a technique, for example, shown in FIGS. 6 and 7 and, in this example, an exhaust gas exhausted from an engine (not shown) is purified through the use of a three-way catalyst (TWC) 102 provided in an exhaust passage 101 and then discharged or released to the atmosphere.
In addition, in the exhaust passage 101, O2 sensors 103 and 104 are provided at an entry and exit of the three-way catalyst 102 so as to detect an air-fuel ratio of an exhaust gas flowing into the three-way catalyst 102 and an air-fuel ratio of an exhaust gas exiting from the three-way catalyst 102, respectively.
Still additionally, this three-way catalyst 102 contains an oxygen storage component (hereinafter referred to as an “OSC adsorbent”), thereby adsorbing oxygen contained in the exhaust gas.
Still additionally, it is known that the degree of degradation of the three-way catalyst corresponds to the degree of the OSC adsorbent, and the technique shown in FIGS. 6 and 7 also employs a method of detecting the degree of degradation of the three-way catalyst 102 by detecting the degree of degradation of the OSC adsorbent.
A description will be given hereinbelow of this method. First of all, a fuel injection quantity and others of an engine are varied periodically, thereby periodically varying an air-fuel ratio to be detected by the upstream side O2 sensor 103 (see mark A1 in FIG. 6).
At this time, if the OSC adsorbent functions normally without degrading, when the exhaust gas reaches an oxygen excess atmosphere (lean), the OSC adsorbent adsorbs the oxygen in the exhaust gas while, when the exhaust gas falls into an oxygen shortage atmosphere (rich), the oxygen adsorbed by the OSC adsorbent is discharged into the exhaust gas. That is, owing to this OSC adsorbent, the variation of the air-fuel ratio is suppressible (see mark A2 in FIG. 6).
On the other hand, in a case in which the OSC adsorbent falls into a degradation condition, difficulty is encountered in sufficiently adsorbing and discharging oxygen through the use of the OSC adsorbent, which makes it difficult to suppress the variation of the air-fuel ratio occurring on the upstream side of the three-way catalyst 102 (see marks A3 and A4 in FIG. 7).
As described above, in the example shown in FIGS. 6 and 7, an estimating the degree of degradation of the three-way catalyst 102 can be made on the basis of a difference in detection value between the upstream side O2 sensor 103 and the downstream side O2 sensor 104 in the case of periodically varying the exhaust air-fuel ratio.
Meanwhile, as another example of technologies for the estimating the catalyst purification performance, there exists a technique disclosed in Japanese Patent Laid-Open No. HEI 6-81635 (patent document 1).
This patent document 1 discloses a technique to the effect that an O2 sensor is provided on each of the upstream and down stream sides of a catalyst and, at the fuel return after the reduction fuel cut, an estimating the degradation of the catalyst is made on the basis of a lag of response time until each of the values measured by these O2 sensors exceeds a fixed value.
The technique disclosed in the patent document 1 also employs a technique similar in principle to the method of estimating the degradation of the catalyst on the basis of the degree of degradation of the OSC adsorbent as described with reference to FIGS. 6 and 7. In addition, for enhancing the estimation accuracy, the estimating for the degradation of catalyst is made at the fuel return after the fuel cut.
However, in a case in which the degree of degradation of the three-way catalyst 102 is detected through the use of the method shown in FIGS. 6 and 7, there is a problem in that, if a plurality of three-way catalysts 122, 123 and 124 are disposed in series as shown in FIG. 8, difficulty is experienced in individually detecting the degree of degradation of each of the three-way catalysts 122, 123 and 124.
That is, as shown in FIG. 8, the first three-way catalyst 122, the second three-way catalyst 123 and the third three-way catalyst 124 are provided in the exhaust passage 121. Moreover, these first to third three-way catalysts 122, 123 and 124 are arranged in the order of the first three-way catalyst 122, the second three-way catalyst 123 and the third three-way catalyst 124 from the upstream side. Each of the first to third three-way catalysts 122, 123 and 124 contains an OSC adsorbent.
In addition, a first O2 sensor 125 is located on the upstream side of the first three-way catalyst 122, and a second O2 sensor is placed on the downstream side of the first three-way catalyst 122 and on the upstream side of the second three-way catalyst 123. Still additionally, a third O2 sensor 127 is provided on the downstream side of the second three-way catalyst 123 and on the upstream side of the third three-way catalyst 124, and a fourth O2 sensor 128 is put on the downstream side of the third three-way catalyst 123.
Accordingly, in the case of varying the exhaust air-fuel ratio periodically, the periodic variation of the exhaust air-fuel ratio is detectable by the first O2 sensor 125 (see mark B1 in FIG. 8).
Moreover, in a case in which the OSC adsorbent of the first three-way catalyst 122 does not fall into a degradation state, the OSC adsorbent of the first three-way catalyst 122 fulfills its function, thereby reducing the air-fuel ratio variation of the exhaust gas discharged from the first three-way catalyst 122 to the downstream side (see mark B2 in FIG. 8). This can reduce the air-fuel ratio variation detected by the second O2 sensor 126, the air-fuel ratio variation by the third O2 sensor 127 and the air-fuel ratio variation by the fourth O2 sensor 128 (see marks B2, B3 and B4 in FIG. 8). This makes it difficult to make a comparison on the difference between the results of detection by these second to fourth O2 sensors 126, 127 and 128.
That is, in a case in which an upstream side catalyst (for example, the first three-way catalyst 122) is not in a degraded condition, a downstream side catalyst (for example, the second three-way catalyst 123 or the third three-way catalyst 124) cannot detect the degradation.
On the other hand, the catalyst includes various types of catalysts, and it can be hardly said that the technique disclosed in the above-mentioned patent document 1 is applicable to all the catalysts. In particular, difficulty is experienced in detecting the degradation of an HC trap catalyst (unburned fuel component adsorption catalyst) capable of adsorbing HC (hydro-carbon) which is an unburned fuel component in the exhaust gas.
That is, the HC in the exhaust gas, adsorbed by the HC trap catalyst, functions as a reductant to discharge oxygen adsorbed by the OSC adsorbent into the exhaust. Therefore, even if the technique disclosed in the patent document 1 is applied intact, it is difficult to estimate the degradation of the HC trap catalyst.